Sunlight is comprised of a broad range or band of solar radiation in a spectrum spanning short wavelength, high energy, ultra-violet light, through mid-wavelength, visible and near infra-red light, and extending into longer wavelength, lower energy infra-red light. Various semiconductor materials have small enough gaps, called bandgaps, between their valence and conduction energy bands that some level of solar radiation energy will cause electrons in their valence energy bands to transition or jump the bandgap from the valence band to the conduction band, where they can become part of the creation of an electric field and electric current when the semiconductor materials are processed and assembled in a manner that enables such solar energy to electric energy conversion. The size of the bandgap determines how much solar energy is necessary to cause the electrons to transition from the valence band to the conduction band, and semiconductor materials exist or can be made with bandgaps to absorb and convert solar energy from any part of the solar spectrum to electric energy. However, each semiconductor material with its bandgap only absorbs and converts
solar energy to electric energy efficiently in a narrow photon energy range that includes and extends slightly higher than its bandgap energy. If the photon energy in the solar radiation is lower than the bandgap, it will not be absorbed and converted to electric energy in that semiconductor material, but will instead continue to be transmitted through the semiconductor material much as light is transmitted through glass. In other words, semiconductor materials are transparent to solar radiation or light with photon energy less than the bandgap energy, and, except for minor absorption losses that create heat, such lower energy solar radiation will pass through such semiconductor materials and not be converted to electric energy. On the other hand, if the photon energy in the solar radiation is very much higher than the bandgap energy of the semiconductor material, it will be absorbed and cause the electrons to jump the bandgap, thus convert some of such energy to electric energy, but the excess energy over the amount needed for the electrons to jump the bandgap will be thermalized and lost in heat dissipation instead of converted to electric energy. Consequently, for efficient conversion of solar energy from the entire solar radiation spectrum to electric energy, multiple bandgaps distributed throughout the solar spectrum may be needed.
The challenge to implement semiconductor photovoltaic converters with multiple bandgaps distributed throughout the broad solar spectrum has been addressed in a number of ways, including, for example, stacking a plurality of single bandgap photovoltaic converters one on top of another so that light with sub-bandgap energy, i.e., photon energy less than the bandgap of a higher bandgap photovoltaic converter, will pass through that converter to the next lower bandgap converter and, if not absorbed there, perhaps to one or more additional, even lower bandgap converters, until it either gets to a semiconductor material with a low enough bandgap that it will be absorbed and converted to electric energy or gets transmitted out of the system. Another approach has been to include a plurality of subcells with different bandgaps in monolithic, multi-bandgap, tandem, photovoltaic converter devices. Still another approach has been to split the solar spectrum into two or more energy bands and direct each band to a different semiconductor cell with an appropriate bandgap for the energy level of the band that is directed to it.
All of these and other approaches have their advantages and disadvantages. For example, the individual, single bandgap photovoltaic converter cells with different bandgaps stacked together is relatively simple, but reflectance of anti-reflection coatings to prevent reflection of the incident solar radiation is inconsistent and not highly efficient for all wavelengths of light in the solar spectrum, so it is difficult to prevent losses due to reflection at the front face of the top cell, and there are a lot of energy losses associated with multiple surfaces and interfaces and with sub-bandgap absorption, and the like. Monolithic, multi-bandgap, tandem, photovoltaic converters eliminate some surfaces and interfaces, but they have similar front surface and anti-reflection coating issues, lattice matching and mismatching of semiconductor materials imposes constraints on semiconductor materials and bandgaps, and they are more difficult and expensive to make. Split spectrum schemes have the advantage of not having to deal with anti-reflection coatings for the entire solar spectrum, but disadvantages include more complexity with more parts, and more interfaces that generally result in more energy losses. The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.